DSL technology takes advantage of the fact that although a legacy twisted metallic pair (which was originally installed to provide merely a Plain Old Telephone Services (POTS) telephony connection) might only have been intended to carry signals at frequencies of up to a few Kilohertz, in fact such a line can often reliably carry signals at much greater frequencies. Moreover, the shorter the line, the greater is the range of frequencies over which signals can be reliably transmitted (especially with the use of technologies such as Discrete Multi-Tone (DMT), etc.). Thus as access networks have evolved, telecommunications network providers have expanded their fiber optic infrastructure outwards towards the edges of the access network, making the lengths of the final portion of each connection to an end user subscriber (which is still typically provided by a metallic twisted pair) shorter and shorter giving, rise to correspondingly greater and greater bandwidth potential over the increasingly short twisted metallic pair connections—without having to bear the expense of installing new optic fiber connections to each subscriber.
However, a problem with using high frequency signals is that a phenomenon known as crosstalk can cause significant interference reducing the effectiveness of lines to carry high bandwidth signals in situations where there is more than one metallic pair carrying similar high frequency signals in close proximity to one another. In simple terms, the signals from one wire can “leak” onto a nearby line carrying similar signals and appear as noise to the other line. Although cross talk is a known problem even at relatively low frequencies, the magnitude of this effect tends to increase with frequency to the extent that at frequencies in excess of a few tens of Megahertz (depending on the length of the lines in question), the indirect coupling (e.g. from a near end of a second line to a remote end of a first line) can be as great as the direct coupling (e.g. from the near end of the first line to the remote end of the first line).
In order to alleviate the problems caused by cross talk (especially Far End Cross Talk, or “FEXT” as it is known) a technology known as vectoring has been developed in which knowledge of the signals sent over crosstalking lines is used to reduce the effects of the crosstalk. In a typical situation a single DSLAM acts as a co-generator of multiple downstream signals over multiple cross-talking lines and also as a co-receiver of multiple upstream signals from the same multiple cross-talking lines, with each of the lines terminating at a single Customer Premises Equipment (CPE) modem such that no common processing is possible at the CPE ends of the lines. In such a case, downstream signals are pre-distorted to compensate for the expected effects of the cross-talking signals being sent over the neighboring cross-talking lines such that at reception at the CPE devices the received signals are similar to what would have been received had no cross-talking signals been transmitted on the cross-talking lines. Upstream signals on the other hand are post-distorted (or detected in a manner equivalent to their having been post-distorted) after being received at the co-receiver (the DSLAM) in order to account for the effects of the cross-talk which has leaked into the signals during their transmission.
Such vectoring techniques can deal very successfully with situations where the indirect coupling is significantly weaker than the direct coupling. However as the relative strengths of the direct and indirect coupling approach each other, vectoring is less able to function effectively.
WO 2008/005507 describes a system for performing Adaptive Multi-Carrier Code Division Multiple Access (Adaptive MC-CDMA—or AMC-CDMA) which is particularly suited to use in powerline transmission systems where data is transmitted over wires intended for carrying electrical power. Such a system may have many devices connected to one another over a common transmission medium (the power line) for which there is no possibility of sending signals from one transceiver to another over a dedicated respective pair of wires as is the case with a conventional telephone access network. Such a transmission medium suffers from continuously changing conditions. The system AMC-CDMA is therefore adaptive to permit rapid changes in bit loading using the MC-CDMA modulation approach.
US 2005/002441 describes a Multi-Carrier Code Division Multiple Access transmission technique for use in transmitting data over a VDSL link comprising a single twisted copper pair connection between two co-operating VDSL modems. The drawback of using CDMA which is that multiple chips are required for each bit of data to be transmitted is mitigated against by simultaneously sending multiple chips associated with different data bits. For this reason the inventors refer to their scheme as Multi Code Multi-Carrier Code Division Multiple Access (or MC MC-CDMA). Signals being transmitted over other neighboring lines cause FEXT which is treated as noise because it is unrelated to the signal being transmitted over the line connecting the cooperating MC MC-CDMA modems.
“Multiple users adaptive modulation schemes for MC-CDMA” by Tang C et al. (published on 29 Nov. 2004 in the Global Telecommunications Conference, 2004, GLOBECOM '04, IEEE DALLAS, Tex., USA 29 Nov.-3 Dec. 2004, Picataway, N.J., USA pages 3823-3827) XP010758452, ISBN: 978-0-7803-8794-2) describes the use of MC-CDMA under a frequency-selective fading channel (i.e. over the air interface) for wireless systems. The paper investigates this topic and proposes a configuration that not only permits multiple users to employ adaptive modulation, but also leads to an equivalent sub-carrier concept that enables a group of sub-carriers to be represented by an equivalent sub-carrier of a conventional OFDM modem, thereby allowing various powerful bit/power loading schemes originally developed for OFDM to be directly deployed to MC-CDMA.
WO2013026479 applied for by Ericsson proposes a method of transmitting signals, in such a situation (i.e. where an indirect coupling is comparable to a direct coupling for a given line), which involves transmitting signals intended for reception by a single CPE device (a first CPE device) onto both the line directly coupled to the first CPE device and onto a crosstalking line coupled only indirectly to the first CPE device (it being directly coupled to a second CPE device). A Time Division Multiplexing (TDM) method is used to enable data to be sent (in different time slots) to the two respective CPE devices (with data being sent over both wires at the same time to only one of the CPE devices at a time). In order to ensure that the two signals constructively interfere at the receiving CPE device, the same signal as sent over one line is pre-distorted (e.g. to introduce a delay and/or phase change) before being sent over the other to account for changes in the direct vs. the indirect coupled paths.